Many cosmetic and pharmaceutical compositions contain substantially insoluble powders dispersed within fluid bases. For example, sunscreen and color cosmetic compositions often contain insoluble inorganic oxides, such as titanium dioxide and iron oxide, dispersed within water-in-oil emulsions. Such compositions should possess significant suspending power, i.e., they should be capable of suspending the dispersed powders, over a wide temperature range and for an extended period of time, without significant precipitation or separation. Additionally, the compositions should deliver the powders to the skin or hair surface in a consistent and reproducible manner. The compositions should be physically and chemically stable, easy to apply, aesthetically pleasing and nonirritating upon application. Similarly, the compositions should be resistant to removal by water or sweat. Cosmetic chemists and formulators have long attempted to develop compositions with these characteristics.
One way in which both suspending power and stability is reported to be achieved in water-in-oil emulsions is by the use of water-swellable clays. See Yoneyama et al., U.S. Pat. No. 5,015,469. Yoneyama et al. is directed, inter alia, to water- and polyhydric alcohol-in-oil type emulsified compositions containing an oil phase containing a polyoxyalkylene modified organopolysiloxane, a water-swellable clay mineral, a quaternary ammonium salt type cationic surfactant and a silicone oil, water, and a polyhydric alcohol, and optionally containing inorganic powders or pigments.
As mentioned above, a highly desirable method for delivering cosmetic compositions to the skin or hair is in the form of a finely dispersed spray. This form of product delivery offers improved product coverage on the skin or hair and easier application to difficult-to-reach areas. Such a spray is said to have good dispersibility and is desirably delivered using a nonaerosol spray pump, which does not require the use of pressurized containers or special aerosolizing gases.
The ability of such pump-driven delivery systems to deliver a product as a finely dispersed spray is critically dependent upon the viscosity of the composition at the exit port of the pump. As the viscosity of the composition decreases, the spray becomes more dispersed yielding a more desirable delivery. Conversely, as the viscosity increases, the spray becomes less dispersed and more stream-like which is less desirable for the application of cosmetics. However, formulators of powder-containing compositions generally rely on the use of higher viscosities to achieve satisfactory suspending power and stability in their formulations. This presents a problem with respect to delivery.
Moreover, it is well-known in the prior art that stable water-in-oil emulsions are generally difficult to prepare at very low viscosities. For example, the compositions disclosed by Yoneyama et al. (U.S. Pat. No. 5,015,469) are described as having a minimum viscosity of about 2500 centipoise, with the viscosity of the disclosed compositions increasing as the weight percent of water-swellable clay mineral is increased.
There is therefore a clear need for stable, water-in-oil compositions that are readily deliverable to the skin or hair as finely dispersed sprays using nonaerosol spray pumps (hereinafter referred to as "sprayable compositions"), while at the same time possessing a high degree of stability and suspending power.